1. Field of the Invention
This invention relates to a polymer composition that is useful for curing novolac resins. The polymer composition exhibits reduced emissions of ammonia compared to conventional novolac curing agents such as hexamethylenetetramine, and exhibits higher reactivity compared to conventional novolac curing agents. Furthermore, the polymer composition can be stored for extended times without loss of reactivity.
2. Description of the Related Art
For some time, phenolic resins have been used in applications as diverse as molding compositions, surface coatings, adhesives, laminating resins, casting resins and binders. In the plastics molding field, phenolic resins have been a preferred choice as a molding material for precision moldings that must function in hostile environments because phenolic resins react to form cross-linked structures with excellent dimensional, chemical and thermal stability at elevated temperature.
Phenolic resins are thermosetting resins produced by the condensation of an aromatic alcohol with an aldehyde wherein water is produced as a byproduct. Typically, the aromatic alcohol is phenol and the aldehyde is formaldehyde, but substituted phenols and higher aldehydes have been used to produce phenolic resins with specific properties such as reactivity and flexibility. The variety of phenolic resins available is quite large as the aldehyde to aromatic alcohol ratio, the reaction temperature and the catalyst selected can be varied.
Phenolic resins fall into two broad classes: resole (single stage) resins and novolac (two stage) resins. Resole resins are typically produced with a phenol, a molar excess of formaldehyde and an alkaline catalyst. The reaction is controlled to create a non-cross-linked resin that is cured by heat without additional catalysts to form a three dimensional cross-linked insoluble, infusible polymer. In contrast, novolac resins are typically produced with formaldehyde, a molar excess of phenol, and an acid catalyst. The reaction produces a thermoplastic polymer that can be melted but will not cross-link upon the application of heat alone. The resulting novolac thermoplastic resin can be cross-linked by the addition of a novolac curing agent.
Several curing agents for novolac resins are known in the art, including formaldehyde, paraformaldehyde and hexamethylenetetramine. The most common curing agent is hexamethylenetetramine, which reacts upon heating to yield ammonia and cured resin. These curing agents complete the cross-linking reaction to convert a thermoplastic novolac resin to an insoluble infusible state. However, it has been recognized in the art that each of these novolac curing agents has certain disadvantages. For instance, where hexamethylenetetramine or formaldehyde are used to cure a novolac resin, volatile reaction products are emitted during the cure reaction. Specifically, when the curative is hexamethylenetetramine, ammonia evolves during curing of the novolac resin. In addition, novolac curing agents like hexamethylenetetramine typically require curing temperatures as high as 150xc2x0 C. Cure temperatures can be lowered by the addition of acids, but this often introduces other problems such as die staining, die sticking and sublimation of organic acids into the atmosphere.
Melamine resins have also been used as a curing agent for novolac resins. (See, for example, U.S. Pat. No. 5,648,404.) However, there are also disadvantages in the use of melamine resins as a curing agent. For instance, it has been recognized that melamine resins typically require either an acid catalyst or elevated temperatures to cure a novolac resin. Also, melamine resin curing agents tend to cure novolac resins more slowly than hexamethylenetetramine and tend to produce a lesser extent of cure, and frequently produce formaldehyde as a side reaction.
Resole resins have also been used as a curing agent for novolac resins. (See, for example, U.S. Pat. No. 4,745,024.) However, there are significant drawbacks with resole resins. First, resole resins have a limited shelf life which can severely limit product reactivity after a resole resin has been stored for a period of time as short as three months. In addition, resole resins have an even shorter shelf life when mixed with a novolac resin. The limited shelf life is caused by self-condensation in the resole resin wherein phenolic nuclei are bridged by methylene groups. Accordingly, resole resin curing agents may not be suitable for manufacturers that wish to purchase resole curing agents in large quantities and place the resole curing agents in inventory for later use. A second drawback with resole resins is that they may contain significant free phenol and formaldehyde levels that may present environmental concerns for some manufacturers. Specifically, conventional resole resins typically contain 4-6% free phenol and may contain free formaldehyde levels of approximately 1%. A final disadvantage of using resole resins to cure a novolac resin is that a fairly large amount of resole is required to achieve a reasonable crosslink density. Typical formulas are 50% resole or more.
Because of the recognized disadvantages with formaldehyde, hexamethylenetetramine, melamine resin and resole resin novolac curing agents, there has been a search for alternative novolac resin curing agents. One proposed alternative novolac resin curing agent is a benzoxaxine polymer disclosed in U.S. Pat. No. 5,910,521. However, it is believed that this polymer may not present an optimum cost effective alternative to known novolac curing agents.
Therefore, there is a continued need for other alternative novolac resin curing agents that emit limited volatile compounds, such as ammonia, during the novolac cure reaction, that have low levels of free phenol and formaldehyde, that have extended shelf life, and that have lower temperatures for cure activation.
The foregoing needs in the art are met by a polymer composition in accordance with the present invention that can be used to cure a novolac resin at a lower temperature than conventional novolac curing agents and with only limited emissions of volatile compounds. A polymer composition in accordance with the present invention also retains reactivity over extended storage times.
A polymer composition in accordance with the present invention is useful for curing a novolac resin, and comprises the product of mixing and reacting in aqueous solution a phenolic monomer and an aldehyde in the presence of a basic catalyst to form an intermediate resin, and reacting the intermediate resin with an amine to form the polymer composition. The aldehyde to phenolic monomer molar ratio in the reaction should be at least about 1.0:1, and preferably is at least about 2.0:1. The preferred amine is hexamethylenetetramine, and it is preferred that the hexamethylenetetramine to phenolic monomer molar ratio be at least about 0.12:1. In a preferred version of the invention, the aldehyde is formaldehyde, and the phenolic monomer is selected from the group consisting of phenol, substituted phenols, and mixtures thereof, with phenol itself being the most preferred phenolic monomer.
The unreacted phenolic monomer in a polymer composition in accordance with the present invention may be adjusted to levels less than about 3.0 weight percent. Preferably, the unreacted phenolic monomer in the polymer composition is less than about 1.0 weight percent, and most preferably, the unreacted phenolic monomer in the polymer composition is less than 0.5 weight percent. Without intending to be bound by theory, it is believed that by reacting a phenolic monomer with a molar excess of an aldehyde and an amine, the amine keeps the reaction mixture from quickly gelling thereby allowing higher levels of the aldehyde to be introduced into the reaction mixture. The increased levels of aldehyde serve to lower the free phenol levels.
The low unreacted phenolic monomer levels of the polymer composition can be particularly advantageous for manufacturers that presently use conventional resoles for curing novolac resins. For instance, it is well known that conventional resole curing agents typically contain 4-6% free phenol and that phenol emissions and discharges are regulated by certain environmental protection agencies. When a novolac resin is cured with a resole, the free phenol may be liberated as a gas during curing, or may be discharged into process water in the case of slurry molding processes. As a result, manufacturers often must use pollution control equipment to control phenol emissions and discharges and comply with various regulations. By replacing a conventional resole with the polymer curing agent of the present invention, a manufacturer may be able to comply with environmental regulations without the use of pollution control equipment. However, it should be appreciated that the polymer composition is also effective at curing novolac resins when the unreacted phenolic monomer in the composition is greater than about 3.0 weight percent.
The unreacted aldehyde in a polymer composition in accordance with the present invention may be adjusted to levels less than about 0.1 weight percent. This is also particularly advantageous in that the formaldehyde levels in many products are subject to various environmental regulations including reporting requirements. By replacing a conventional resole with the polymer curing agent of the present invention, a manufacturer may be able to more easily comply with certain environmental regulations. The reduced levels of phenolic monomer and formaldehyde in the polymer may also reduce the odors emitted by products molded with a composition including a novolac resin and the polymer curing agent.
In addition, the polymer of the invention, unlike conventional resole curing agents, has an indefinite shelf life in both the pure form and when blended with novolac resins. Experimental data indicates that samples of the polymer will remain essentially unchanged for over a year. Without intending to be bound by theory, it is believed that by reacting a phenolic monomer with a molar excess of an aldehyde and an amine, the available crosslinking sites in the polymer are depleted, yielding a substantially inactive polymer. An unexpected result is that the substantially inactive curing agent polymer of the invention lowers the cure temperature of a novolac resin compared to novolac resins cured with other conventional novolac resin curing agents. Another unexpected result is that the amount of curing agent required for cure is less than the amount of conventional resole resin required for a similar degree of cure. This is particularly advantageous in that resole resins can be quite expensive due to the smaller batch sizes used in production and lower yields.
In one application of the curing agent polymer of the present invention, an effective amount of the polymer is mixed with a solid novolac resin or a solution of a novolac resin to replace conventional curing agents. An effective amount is the amount of the crosslinking polymer composition required to yield the desired extent of cure. The resulting curable novolac composition may then be cured in the conventional manner. Optionally, the curable novolac composition prepared with the curing agent polymer composition of the present invention may include an additional curing agent that supplies methylene bridging groups, such as hexamethylenetetramine. In this application, the polymer of the invention also acts as a cure accelerator and a flow modifier.
In another application of the curing agent polymer of the present invention, an effective amount of the polymer is mixed with a solid or liquid novolac resin and at least one filler to produce a curable composition. Novolac resins and fillers commonly used in curable compositions are suitable for use in this application of the polymer of the invention. Non-limiting examples of fillers are glass fibers, carbon fibers, graphite fibers, ceramic fibers, mica, clays, talcs, cellulosic fibers such as wood flour and cotton fibers, organic fibers such as aromatic polyamide fibers, as well as ground, cured phenolic resins and compounds.
It is therefore an advantage of the present invention to provide a curing agent for novolac resins.
It is another advantage of the present invention to provide a curing agent for novolac resins that lowers the cure temperature of the novolac resin compared to novolac resins cured with other conventional novolac resin curing agents.
It is a further advantage of the present invention to provide a curing agent for novolac resins that does not lose reactivity when stored alone or when stored admixed with a novolac resin.
It is yet another advantage of the present invention to provide a curing agent for novolac resins that is stable when the curing agent and a novolac resin are blended before curing and may be used with molding compositions including conventional fillers.
It is still another advantage of the present invention to provide a curing agent for novolac resins that releases limited quantities of ammonia when the curing agent and a novolac resin are blended together and cured.
It is a further advantage of the present invention to provide a curing agent for novolac resins that contains extremely small amounts of free phenol.
It is yet another advantage of the present invention to provide a curing agent for novolac resins that contains extremely small amounts of free formaldehyde.
It is still another advantage of the present invention to provide a curing agent for novolac resins that assists in the production of a cured novolac resin that releases limited odors.
These and other features, aspects, and advantages of the present invention will become better understood upon consideration of the following detailed description, drawings, and appended claims.